Q. What can you tell me about the history of purple glass?

Hello! Thank you for your question. Purple glass is made from the metal oxide manganese, which is added to the batch ingredients. Many glass manufacturers, such as the Imperial Glass Company, produced purple glass. It is also possible that your glass piece might once have been clear but turned purple when exposed to the sun.

According to another Museum staff member who has researched this subject:

Many glassmakers through the centuries have attempted to produce clear, colorless glass. Impurities, especially iron oxide, in the batch ingredients that were melted to make the glass often resulted in glass that was greenish instead of the desired "water clear."

An interesting characteristic of colorless glasses which contain manganese dioxide as a decolorizer is their tendency to turn different shades of purple when exposed to the rays of the sun or to other ultra-violet sources. It is a photochemical phenomenon that is not yet perfectly understood. It is generally accepted that the ultra-violet light initiates an electron exchange between the manganese and iron ions. This changes the manganese compound into a form that causes the glass to turn purple.

It was in the mid 19th century that manganese dioxide, popularly called "glassmaker's soap," began to be used by American glass manufacturers as a decolorizer. By including a small amount of this ingredient in the melt, they could produce glass that appeared virtually colorless. An 1899 publication by Benjamin Biser remarked, "The especial use of manganese in glass is to mask or neutralize the greenish color imparted to the glass by the protoxide of iron. Manganese imparts to glass a pink or red tint, which being complementary to green, neutralizes the color and permits the glass to transmit white light. Pellat refuted this theory, and claimed that the green tint of iron was not neutralized by the pink of manganese, and thus subduing it; but by the iron taking another charge of oxygen from the manganese and becoming per-oxide of iron, and producing a reddish yellow tint, while the protoxide produces a green tint."

Glass scientists today generally agree with Apsley Pellat, explaining that an ion exchange between the iron and the manganese molecules changes the observed color of the glass.

In the early 20th century, changes in manufacturing processes, as well as more pure batch materials, dictated different ways to decolorize glass, and the use of manganese oxide for this purpose dwindled.

It is impossible to determine the age of your glass without having an expert examine it, but you can send pictures of your glass object to our curatorial department. They may be able to provide you with additional information or point you in other directions. You can contact them through their online form (https://www.cmog.org/glass-questions).

Please do not hesitate to contact us with your glass-related questions in the future!